An example from related art, Japanese Patent Laid-open No. 2002-242631, a valve gear is equipped with a camshaft supported in a freely rotatable manner via a pair of main bearings at a camshaft holder fastened to a cylinder head using a bolt, a pair of valve lifters for opening and closing a pair of intake valves, a rocker shaft fixed and supported at the camshaft holder having an axis parallel with the axis of rotation of the camshaft, and a rocker arm supported in a freely swinging manner at the same rocker shaft.
A pair of intake cams having the same prescribed cam surface, and a single exhaust cam having a prescribed surface positioned at a substantially central position between the intake cams are then formed at the camshaft. The pair of intake cams make sliding contact with the top surfaces of the pair of valve lifters, the valve lifters slide according to the cam surface, and the pair of intake valves open and close at a prescribed opening and closing timing by a prescribed lift amount.
A roller making rolling contact with one exhaust cam is supported in a freely rotating manner at one side of the rocker arm camshaft, and a branch dividing into two is formed at the other side. The end of each branch makes contact with the upper end surfaces of the stems of the pair of exhaust valves, and both of the exhaust valves open and close at a prescribed timing and by a prescribed lift amount according to the exhaust cam surface.
With related decompression devices, a centrifugal weight and a decompression cam linking with this centrifugal weight are provided at the outer side of one of the bearings supporting the camshaft, a decompression cam arm making contact with one end of a slipper so as to perform sliding driving is provided at one end of the decompression cam, with one end of the rocker arm branching section being driven by the other end of the decompression arm, so that the exhaust valve is driven so as to be opened and closed for decompression.
With decompression devices of the related art, because the decompression cam is arranged on the outside of the bearings, a decompression cam extending to the tip of the branching part for driving the exhaust valves to open and close from the decompression cam is required, the structure becomes complex, and the cylinder head also becomes large. The present invention therefore sets out to provide a decompression device that is simple in structure and small in size.